Electric cable having a nonmigratory insulating compound



Feb. 19, 1952 A. KING ELECTRIC CABLE HAVING A NONMIGRATORY 4' Lead sheaf/7.

paper screen.

Conauafor 5 Ju/e serving.

7 Jufe serving.

6726/ Wine armour/n9 Inventor 5"; M v M Attorney Patented Feb. 19, 1952 ELECTRIC CABLE HAVING A NONMIGBA- TORY INSULATING COMPOUND Albert King, London, England, assignor to Britlsli Insulated Callenders Cables Limited, London, England, a British company Application January 7, 1946, Serial No. 639,699

In Great Britain January 18, 1945 7 Claims.

This invention'relates to electric cables of the impregnated dielectric type of which the wrapped paper dielectric impregnated with mineral oil or oil-rosin compound is typical. In installations in which such cables are placed vertically, as in mine shafts, or on steep gradients, there is, with a fully impregnated cable as at present manufactured, the danger that migration of compound from the top of the cable will take place leaving it weakened electrically and at the same time causing the distention of the sheath of the lower part, or other structural disturbance at the lower end of the length of cable.

By stating that the cable is fully impregnated it is intended to imply that the cable is as full of impregnant as is possible, the paper being impregnated and the spaces between the turns and layers thereof being filled with compound, the object being to ensure that the possibility 01' the existence of voids within the sheath which surrounds the dielectric is reduced to a minimum.

In view of the difiiculties above indicated it has not been hitherto practicable to employ a fully impregnated cable in a vertical position or on a very steep gradient for a run of more than, say, 50 feet and various expedients such as partial draining of a mass-impregnated cable, or the use of special ancillary apparatus have been devised for dealing with the problem. The expedient of draining has only been partially successful in that it entails an increase in the insulation thickness to allow for a similar factor of safety when compared with a fully impregnated cable. The present invention avoids problems due to migration of impregnant and avoids any increase in size of the cable and any sacrifice of completeness of impregnation.

In this improved cable we employ an impregnant which, at the temperature of impregnation, is sufliciently fluid for penetration of the dielectric by the usual methods (so that the cable can be fully impregnated) and at the maximum working temperature of the cable will not flow along the cable to any detrimental extent while at the minimum temperature at which the cable will be handled the impregnant will not be set so hard as to prevent the relative movements of the turns of paper or other fibrous material which are necessary to accommodate the bending of the cable. At the same time the impregnant has good electrical properties suitable for high dielectric stress. 7

The principal, and in some cases the only ingredient of the impregnant, is microcrystalline or higher. By the term microcrystalline petro leum wax is meant wax of the petrolatum group or petroleum ceresin group of petroleum waxes (as defined and classified in the Journal of the Institute of Petroleum, vol. 29, No. 240, 1943, pp. 361-3), which has a relatively small crystal size compared with that of waxes of the paraflin wax group. Such wax is currently known as microcrystalline wax or amorphous wax. As indicated above, the impregnant may consist entirely of microcrystalline petroleum wax as above defined. It may, however, be combined with other materials to a varying extent according to the properties to be obtained but in all cases it will constitute not less than 55% by weight of the impregnant.

The other ingredient or ingredients of the compound are cable impregnating mineral oil, or a selected plasticizer for the wax, or a mixture of such oil and plasticiser.

By a cable impregnating mineral oil we mean a mineral oil suitable for cable dielectric impregnation. The grades of mineral oil suitable for this purpose are generally known amongst manufacturers of electric cables and are discussed, for instance, in the symposium of papers on insulating oils in the Journal of the Institution of Electrical Engineers, part II, pages 3-64, vol. 90, 1943.

The proportion of material added to the wax is dependent largely upon the lowness ot the temperature at which handling of the cable is to take place. Both the mineral oil and the plasticiser have the efiect of maintaining at low temperatures the freedom of movement needed for bending; for the lowest handling temperatures 45% of such ingredients in the compound may be employed. They have, however, in addition, the effect of reducing'the temperature at which flow of the compound begins to take place along the cable. The plasticisers have this efiect less than in oil. They weaken the crystal structure to a smaller degree at these higher temperatures. On the other hand, they may have less good electrical properties than the most suitable mineral oils. The choice of the combination of ingredients is, therefore, a matter of carefully balancing the different requirements of any particular case.

The plasticisers which may be used are polyisobutene, of the grades having average molecular weight from 20,000 to 100,000 and rosin.

Compositions given by way of example are as petroleum wax having a melting point of C. follows:

Parts by weight Microcrystalline wax, melting point 85 C '10 Hollow core cable oil 30 This compound has a melting point of 84 C.

Parts by weight Microcrystalline wax, melting point 85 C 55 Polyisobutene, molecular weight 20,000 10 Mineral oil, viscosity 900 redwood seconds at This compound has a melting point of 83 C.

Parts by weight Microcrystalline wax, melting point 82 C. 65 Mineral oil, viscosity 600 Redwood seconds at 60 C 20 Rosin (American M. grade) 15 (4) Parts by weight Microcrystalline wax, melting point 80 C. 99

Polyisobutene, molecular weight 20.000 1 This compound has a melting point of 80 C.

Parts by weight Microcrystalline wax, melting point 82 C. 98

Polyisobutehe. molecular weight 20,000 2 This compound has a melting point of 82 C.

Parts by weight Microcrystalline wax, melting point 85 C. 95 Rosin refined as in Example 4 5 This compound has a melting point of 84 C.

The melting points given above are those obtained by the standard method of test for melting point of petrolatum, designated D 127-30 and adopted by the American Society of Testing Materials, which is described on pages 245-6 of Part IIIA of the 1946 book of A. S. T. M. standards. These compounds are prepared by mixing hot and are sufliciently fluid at the ordinary impregnating" temperatures, about 130 C. to ensure complete impregnation of the dielectric. At temperatures below the melting point given for the compound they are plastic solids which will not flow appreciably in the cable. At a temperature of 0 C. these compounds are still sufiiciently plastic to allow the papers of the dielectric to slide relative to one another without damage. The electrical properties of the compounds and of cable paper impregnated with them are good and of a sufliciently high order to be satisfactory for cables with voltage rating of 33 kv. and higher.

The accompanying drawing which is not to scale shows in cross-section an example of a three core paper insulated mine shaft cable in which my invention may be embodied. Each core comprises a sector shaped, stranded copper wire conductor I on which is a built up body of insulation consisting of helically lapped paper tapes 2. Surrounding each insulated conductor is a conductive screen 3 of metallized paper and enclosing the laid up cores is a lead sheath 4. In

laminated paper body on each conductor and impregnating the paper of which the body is built up is a normally plastic solid comprising microcrystalline petroleum wax having a melting point of at least C. and cable impregnating mineral oil, the proportion of wax amounting to at least 55% by weight of the whole. Over the lead sheath there is a protective layer 5 of impregnated jute on which is laida steel wire armouring 6 which in turn is covered by a second protective layer 1 of impregnated jute.

What I claim as my invention is:

1. An impregnated paper-insulated electric cable comprising a conductor, an intersticed laminated body of paper surrounding said conductor, an impervious sheath surrounding said laminated body, and a normally plastic solid comprising micro-crystalline petroleum wax having a melting point of at least 80 C. in the interstices of said body and impregnating the paper of which said body is formed.

2. An impregnated paper-insulated electric cable comprising a conductor, an intersticed, laminated body of paper surrounding said conductor, an impervious sheath surrounding said laminated body, and a normally plastic solid in the interstices of said body and impregnating the paper of which said body is formed, said normally pastic solid consisting of micro-crystalline petroleum wax having a melting point of not less than 80 C. and of cable impregnating mineral oil, the proportion of said micro-crystalline petroleum wax in the normally plastic solid being not less than 55% by weight.

3. An impregnated paper-insulated electric cable comprising a conductor, an intersticed, laminated body of paper surrounding said conductor, an impervious sheath surrounding said laminated body, and a'normally plastic solid in the interstices of said body and impregnating the paper of which said body is formed, said normally plastic solid consisting of micro-crystalline petroleum wax having a melting point of not less than 80 C. and of polyisobutene having a molecular weight of from about 20,000 to 100,000, the proportion of micro-crystalline petroleum wax in the normally plastic solid being not less than,55% by weight.

4. An impregnated paper insulated electric cable comprising a conductor, an intersticed, laminated body of paper surrounding said conductor, an impervious sheath surrounding said laminated body, and a normally plastic solid in the interstices of said body and impregnating the paper of which said body is formed, said v normally plastic solid consisting of micro-crystalline petroleum wax having a melting point of not less than 80 C. and of a mixture of cable impregnating mineral oil and polyisobutene having a molecular weight of from about 20,000 to 100,000, the proportion of micro-crystalline petroleum wax in the normally plastic solid being not less than 55% by weight.

5. An impregnated paper insulated electric cable comprising a conductor, an intersticed, laminated body of paper surrounding said conductor, an impervious sheath surrounding said laminated body, and a normally plastic solid in the interstices of said body and impregnating the paper of which said body is formed, said normally plastic solid comprising micro-crystalline petroleum wax having a melting point of not less than 80 C. and of rosin, the proportion of micro- 5 crystalline petroleum wax in the normally plastic solid being not less than 55% by weight.

6. An impregnated paper insulated electric cable comprising a conductor, an intersticed, laminated body of paper surrounding said conductor, an impervious sheath surrounding said laminated body, and a normally plastic solid in the interstices of said body and impregnating the paper of which said body is formed, said normally plastic solid consisting of micro-crystalline petroleum wax having a melting point of not less than 80 C. and of a mixture comprising cable impregnating mineral oil and rosin, the proportion of microcrystalline petroleum wax in the normally plastic solid being not less than 55% by weight.

7. An impregnated paper insulated, electric cable comprising a conductor, an intersticed, laminated body of paper surrounding said conductor, an impervious sheath surrounding said laminated body, and a normally plastic solid in the interstices of said body and impregnating the paper of which said body is formed, said normally plastic solid having a melting point of about 84 C. and consisting of about 70 parts by weight of micro-crystalline wax having a melting point of about 85 C. and of about 30 parts by weight of hollow cable oil.

ALBERT KING.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Number Name Date Thomas Jan. 11, 1921 Thompson Mar. 30, 1937 Otto et a1. June 22, 1937 Zender June 14, 1938 Abrams et al Dec. 27, 1938 Brazier et al Jan. 9, 1940 Rosen June 18, 1940 Eckel Oct. 1, 1940 Scott et a1. July 14, 1942 Abrams et a1. May 9, 1944 Bennett Jan. 30, 1945 Pearsall Oct. 23, 1945 Mack Nov. 13, 1945 FOREIGN PATENTS Country Date France Oct. 8, 1934 OTHER REFERENCES 

1. AN IMPREGNATED PAPER-INSULATED ELECTRIC CABLE COMPRISING A CONDUCTOR, AN INTERSTICED LAMINATED BODY OF PAPER SURROUNDING SAID CONDUCTOR, AN IMPERVIOUS SHEATH SURROUNDING SAID LAMINATED BODY, AND A NORMALLY PLASTIC SOLID COMPRISING A MICRO-CRYSTALLINE PETROLEUM WAX HAVING A MELTING POINT OF AT LEAST 80* C. IN THE INTERSTICES OF SAID BODY AND IMPREGNATING THE PAPER OF WHICH SAID BODY IS FORMED. 